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Since the 1950s mathematicians have been trying to sort out exactly why some animals, like tigers and zebras, have stripes that are oriented perpendicularly to their spines, while others, like the zebrafish, have stripes that are parallel. Now Harvard researchers are proposing a mathematical model in the journal Cell Systems that, while not quite nailing the "nitty-gritty details of exactly what molecules or cells are mapping the formation of stripes," at least "can describe what happens in stripe formation" more generally. One surprise finding? That only a small change to the model will switch stripes from vertical to horizontal. What triggers that change, however, remains elusive.

The mathematical equation focuses on predicting three "main perturbations" that can influence stripe orientation, the scientists report in a news release on Eureka Alert—change in "production gradient" to affect the density of stripe patterns, change in "parameter gradient" to influence stripe formation, and change in the direction of the stripe's origin, be it molecular, cellular, or mechanical. "We wanted a very simple model in hopes that it would be big picture enough to include all of these different explanations," says lead author Tom Hiscock of Harvard Medical School. The model is an extension of mathematician Alan Turing's earlier research into stripes, reports the Christian Science Monitor, but until the math works in living things, the latest model out of Harvard remains theoretical. (Did you know there's a mathematical model to predict happiness?)